With the development of an advanced information society and the arrival of a multimedia society, electromagnetic interference caused when electromagnetic waves generated from electronic devices have adverse effects on other devices and the human body is becoming a serious social issue. In the background of the increasingly worsening electromagnetic wave environment, electromagnetic wave shielding materials have been developed as a means of protection against electromagnetic waves.
Conventionally used electromagnetic wave shielding materials are ones obtained by covering, with a metal coating, a woven or knitted fabric such as a woven fabric made of a light-weight and flexible fiber, for example a synthetic polymer fiber. Examples of the means for forming a metal coating on a fiber include the vacuum vapor deposition method, the sputtering method, and the electroless plating method. The woven fabric woven with a metal-coated fiber as described above has problems that it is difficult to fabricate thin ones, shaping stability is low, and processability is not good.
A hybrid car and electric automobile society has arrived in recent years, and these automobiles use a direct current of 400 to 600 V. This direct current has switching noise to a large degree and is a source of low frequency noise. However, thinness and light-weight property are not satisfied by electromagnetic wave absorbers for electromagnetic waves in the vicinity of 1 GHz which could affect carcinogenicity and cause malfunction of electronic devices and for low frequency electromagnetic waves of 10 MHz or less which could affect the human body.
A material selected in advance from electromagnetic wave shielding materials according to the absorption frequency is used in the electromagnetic wave absorber having an ability of absorbing electromagnetic waves, and a method is used in which the thickness thereof is changed in order to satisfy conditions such as the desired absorption frequency and the maximum absorption amount at that frequency. For example, a λ/4 type electromagnetic wave absorber requires a thickness of about 50 cm for an electromagnetic wave of 100 MHz. A thickness greater than that is required at lower frequencies. Moreover, since backing with a metal plate is required, practical use is difficult.
For the purpose of reducing the weight of the vehicle body of a hybrid car or an electric automobile, a more light-weight electromagnetic wave absorber capable of absorbing a wide range of frequencies even including electromagnetic waves of low frequency, for example 1 GHz or less and furthermore 10 MHz or less is required.
In addition, with the size reduction and weight reduction of motor and inverter, there is a demand for electromagnetic wave absorbing materials having high heat resistance which can withstand the heat generation of conducting wire due to a large high-frequency electric current flowing from the inverter to the electric motor. There is a demand for materials with high heat resistance because of a large temperature rise of equipment particularly in electric/electronic devices such as large rotating machines to which a high voltage is applied.
As an electrical insulator and a thin leaf structure material, high heat resistance aramid paper is widely used as an electrically insulating material of the rotating machine described above (generator, electric motor), a transformer field, and electric/electronic devices, and studies have been conducted so far on the use of this aramid paper as an electric field relaxing material by giving a certain degree of conductivity.
Japanese Patent Application Publication No. Sho 51-47103 and Japanese Patent Application Publication No. Sho 57-115702 disclose paper using an aramid fibrid and a carbon fiber or a metal fiber. In addition, Published Japanese Translation of PCT International Application Publication No. 2008-542557 discloses an electromagnetic wave suppression sheet which is composed of an aramid short fiber, an aramid fibrid, and a conductive filler such as a carbon fiber.
However, none of them aims at the electromagnetic wave absorption described above and thus satisfies characteristics which have important relationships as electromagnetic wave absorption characteristics, for example dielectric loss characteristics and scattering characteristics.